1. Field of the Invention
The present invention relates to a decoder for a digital TV receiver and more particularly for a digital TV to improve the picture quality of a video when a Standard Definition class TV receiver receives a High Definition class video signal.
2. Background of the Related Art
Generally, a digital TV receiver receives and decodes a video signal compressed in the MPEG2 format at a decoder to display the video on a screen. Depending upon the number of picture elements (pels) in relation to the resolution and image reproductivity or picture quality, a digital TV supports two classes of video signals, namely the High Definition TV (HDTV) and the Standard Definition TV (SDTV). The HDTV class video signal has a maximum capability of 1920 pels/1080 lines and the SDTV class video has a capability of 704 pels/480 lines. For purposes of explanation, the capability will be limited to a HDTV class signal of 1920 pels/1080 lines and a SDTV class signal of 720 pels/480 lines.
A conventional decoder for a digital TV receiver will be explained with reference to the figures. A widely used format is a ratio of three components of luminance information Y and chrominance information Cb and Cr in a horizontal (or scanning) line on a TV screen or the like, represented as 0:0:0. FIG. 1A shows a 4:4:4 video format, FIG. 1B shows a 4:2:2 video format, and FIG. 1C shows a 4:2:0 video format. In the figures, the cross represents a luminance signal Y and the circle represents a chrominance signal Cb or Cr.
Particularly, the luminance indicates the extent of brightness of an image and the luminance of a pel is represented with 8 bits according to the ITU-R BT.601 recommendation. The chrominance or color difference is the information on a color of an image and is also represented by 8 bits according to the ITU-R BT.601 recommendation. Thus, a total of 24 bits are assigned to one pel. However, because the human eye is not very sensitive to the small variations in color, some color information is frequently left out in the representation of the video.
Particularly, FIG. 1A shows the video format 4:4:4 in which no color information is left out. FIG. 1B shows the video format 4:2:2 in which half of the color information in a horizontal direction is left out. FIG. 1C shows the video format 4:2:0 in which half of the color information in both the horizontal and vertical direction is left out. Namely, the 4:2:2 format has color information which is half of luminance information, and the 4:2:0 format has color information which is quarter of luminance information.
In a digital TV receiver, one digital pel is typically expressed by 8 bits and every macro block has 16.times.16 pel data. The decoder processes a bit stream corresponding to each class inclusive of Discrete Cosine Transform (DCT) coefficients and motion vector information. FIG. 2 shows a block diagram of a conventional decoder for a digital TV receiver including a Variable Length Decoder (VLD) 1 for variable length decoding of a received bit stream data to provide DCT coefficients and motion vectors; an Inverse DCT (IDCT) 2 for inverse discrete cosine transforming the DCT coefficients to decode the DCT coefficients into spatial pel values; an adder 3 for adding data motion compensated at a motion compensated predictor 4 to the decoded data, to restore a video data; a frame memory 5 for storing the restored video data in a 4:2:0 video format; and an upsampling unit 6 for upsampling the data stored in the frame memory 5 into a 4:2:2 video format to output the video to a display unit. The motion compensated predictor 4 compensates the video data stored in the frame memory 5 using the motion vectors from the VLD 1, and forwards the compensated data to the adder 3.
Having a capability to receive and decode a HD class video data, a HD class digital TV receiver has no problems receiving and decoding a SD class video data. However, a SD class digital TV which is adapted to receive and decode a SD class video data cannot receive and decode a HD class video data. Nevertheless, in order to achieve full compatibility, the SD class digital TV receiver should also have the capability to receive and process the HD class video class. Thus, a decoder for the SD class digital TV receiver has been adapted to carry out a SD class data processing on a HD class data by a process known as down-converting, which is essentially a down sampling or filtering/decimation.
FIG. 3 shows one example of a decoder in the related art for a SD class TV receiver to carry out the down-converting process by receiving, decoding and displaying a HD class data. In FIG. 3, an 8.times.8 block data is decoded by reducing the data into a 4.times.4 block data. An 8.times.8 block data 41 is inverse discrete cosine transformed and downsampled in the horizontal and vertical directions at an IDCT/down-sampling unit 42 for 8 pels to reduce the 8.times.8 block into a 4.times.4 block. Particularly, 16 coefficients, indicated as black dots in the data block 41, are selected from the upper left hand corner and subjected to the IDCT to obtain a down-sampled video data. The remaining coefficients, indicated as white dots are discarded. There are numerous other ways in selecting the coefficients.
In FIG. 3, the 4.times.4 down sampled video data is added to the motion compensated data at an adder 43 and stored in a frame memory 44 (or field memory). In order to process motion vectors corresponding to the down-sampled video data, the video data in the frame memory 44 is upsampled in horizontal/vertical directions at an upsampling unit 45 to an 8.times.8 data. The upsampled video data is motion compensated by the motion vectors at a motion compensated predictor 46, downsampled in horizontal/vertical directions at a downsampling unit 47 back to a4.times.4 data, and added to the adder 43 to obtain a video block downsampled from 8.times.8 data to 4.times.4 data. The downsampled video data in finally stored in the frame memory. To present the data, the data from the frame memory 44 is converted into the 4:2:0 format at a format converter 48 and converted into the 4:2:2 format at an upsampling unit 49 before presentation.
FIG. 4 shows another example of a decoder in the related art for a SD class TV receiver to carry out the down-converting process by receiving, decoding and displaying a HD class data. In FIG. 4, an 8.times.8 block data is decoded by reducing the data into an 8.times.4 block data. An 8.times.8 block data 51 is inverse discrete cosine transformed and downsampled in the horizontal and vertical directions at an IDCT/down-sampling unit 52 for 8 pels to reduce the 8.times.8 block into a 8.times.4 block. Similarly to FIG. 3, after selecting 32 coefficients from the left hand side, indicated as black dots in the data block 41, the remaining coefficients, indicated as white dots are discarded. There are also numerous other ways in selecting the coefficients.
In FIG. 4, the 8.times.4 down sampled video data is added to the motion compensated data at an adder 53 and stored in a frame memory 54 (or field memory). In order to process motion vectors corresponding to the down-sampled video data, the video data in the frame memory 54 is upsampled in horizontal/vertical directions at an upsampling unit 55 to an 8.times.8 data. The upsampled video data is motion compensated by the motion vectors at a motion compensated predictor 56, downsampled in horizontal/vertical directions at a downsampling unit 57 back to an 8.times.4 data, and added to the adder 53 to obtain a video block downsampled from 8.times.8 data to 8.times.4 data. The downsampled video data in finally stored in the frame memory. To present the data, the data from the frame memory 54 is converted into the 4:2:0 format at a format converter 58 and converted into the 4:2:2 format at an upsampling unit 59 before presentation.
FIG. 5 shows yet another example of a decoder in the related art for an SD class TV receiver to carry out the down-converting process by receiving, decoding and displaying a HD class data. FIG. 6 explains an encoding/decoding method for receiving, decoding and displaying a HD class data at an SD class TV receiver.
The intra (I), predicted (P) and bi-directional (B) pictures are downsampled to an 8.times.4 block. The downsampled B picture is converted into a 720 pels/480 lines through a format conversion and encoding before being stored in a memory. The downsampled I and P pictures are first decoded and also converted into a 720 pels/480 lines through a format conversion. The decoded I and P picture data are presented together with the decoded B picture data.
In FIG. 5, an IDCT/downsampling unit 61 inverse discrete cosine transforms and filters/downsamples the received DCT coefficients to convert from 8.times.8 to 8.times.4 blocks. The downsampling is performed for all I, P and B pictures, and the downsampled 8.times.4 data is forwarded to an adder 62. The adder 62 adds the downsampled data to a motion compensated data, and a classifier 63 classifies the data from the adder 62 into I, P and B pictures. The B picture is converted into a 4:2:0 format through a format converting unit 64. A data reduction of the format converted B picture is performed by a data encoding unit 65b, and a memory 66b in a memory 66 stores the B picture. An exemplary bit reduction at the data encoding unit 65b is shown in FIG. 6 and may be applied to a data encoding unit 65a for the I and P pictures. Also, data decoding is a reverse operation of the data encoding.
Referring to FIG. 6, in consideration of a limited memory size, the number of bits is reduced utilizing a correlation between adjacent pels. For example, an 8.times.4 bits are reduced to 14 bits considering the correlation between adjacent pels in a horizontal (or vertical) direction. Particularly, the horizontal (or vertical) pels p1-p4701 of the decoded image pels 700 having 16 bits per two horizontal (or vertical) pels are converted into pl (original value), p2-p1 (a difference between adjacent pels), p3 and p4-p3 (a difference between adjacent pels) 702. Both the differences between adjacent pels p2-p1 and p4-p3 are represented with 9 bits due to a possibility of a negative sign.
Data of the converted pels p1, p2-p1, p3 and p4-p3 are coded using a non-uniform quantization table 703, wherein p1 is coded into an 8 bit value, p2-p1 is coded into a 6 bit value, p3 is coded into an 8 bit value, and p4-p3 is coded into a 6 bit value. The results of the coding 704 are stored in a memory 705, which may be an anchor frame memory 66a or the B frame memory 66b shown in FIG. 5. The data with the reduced bits is thus stored in the B frame memory 66b and may be decoded through a reverse process of the coding process discussed above at a first data decoding unit 71 to restore or reproduce an image.
Depending upon the classifier switching unit 63, a data reduction of the I or P picture may also be performed at an encoding unit 65a by a method discussed above in reference to FIG. 6 and the anchor frame memory 66a in the memory 66 would then store the data reduced I or P picture. Thereafter, the I or P picture would be decoded in a second and third data decoding units 67 and 72.
Particularly, the 8.times.4 data decoded at the second data decoding unit 67 is upsampled in the horizontal direction at a horizontal upsampling unit 68 which receives motion vectors to output an 8.times.8 data. The upsampled 8.times.8 data is output to a motion compensated predictor 69 for motion compensation using motion vector information, and forwarded to a horizontal downsampling unit 70 in an 8.times.8 size. The horizontal downsampling unit 70 downsamples the 8.times.8 motion compensated block in the horizontal direction into an 8.times.4 block and forwards the downsampled data to the adder 62, thereby providing motion compensated I, P and B picture signals in conformity with the 8.times.4 P and B pictures (sizes) downsampled by the IDCT/downsampling unit 61.
By repeating the aforementioned process, the I, P and B frame data are processed, wherein an 8.times.4 data of the I, P and B pictures output from the anchor frame memory 66a and decoded at the third data decoder 72 are converted into a 4:2:0 format at a format converting unit 73 (made to be in conformity with a B frame format). The converted 4:2:0 format data is further converted into a 4:2:2 format at the upsampling unit 74, and output as a final video signal together with the B picture information.
Nevertheless, the described decoder in the related art for an SD class TV receiver to receive, decode, and display an HD class data has problems. First, the storage of a video signal in a 4:2:0 format and upsampling the stored data into a 4:2:2 format due to the memory size does not take into consideration a human's visual sensation characteristic. Since our experiments show that a human is visually more sensitive to a color variation in a vertical direction than a horizontal direction, the decoder in the related art results in a color signal resolution which is too low relative to a luminance signal resolution.
That is, a SD class TV decoder in the related art for receiving, decoding, and displaying a HD class data converts a video data from a 4:4:4 format to a 4:2:0 format and stores the converted data in a memory considering only a memory size. In such case, portions of horizontal and vertical color signals are left out as the video signal is converted from a 4:4:4 format into 4:2:0 format. Accordingly, in the upsampling of the video signal from the 4:2:0 format to the 4:2:2 format or 4:4:4 format, the color signal which is upsampled is not the original color signal left out, but a color signal interpolated around the original selected color signal. Thus, an image reproduced from the converted video signal in the related art results in deterioration of a picture quality with respect to visual sensations.
Second, considering a memory size, the video signal is first converted into the 4:2:0 format before storage and re-converted into the 4:2:2 format before presentation. However, the conversion of the video signal into 4:2:0 format before an internal process results in the reduction of color informationion. Therefore, the conversion from the 4:2:0 format including less color signal to the 4:2:2 format may be a mere re-conversion of a video signal with less color signal, resulting also in a deterioration of picture quality.